Recycling of waste toner derived from exhausted printer cartridges as adsorbent for defluoridation of water

Due to the broad adoption of electronic and electrical equipment and the quick advancement of contemporary innovations in this domain


Introduction
The demand for electrical and electronic gadgets has been expanding rapidly as a result of urbanization, industrialization, and population expansion.Electronic appliances, such as housing electrical appliances and IT/telecommunications devices, have become indispensable to everyday life.They produce a significant quantity of "e-waste"; according to the worldwide e-waste monitor, 53.6 Mt treatment.The objective of this work was to evaluate the adsorption capacity of PCt powder as an adsorbent for the elimination of fluoride from aqueous solutions.The study also investigated the influence of several parameters, such as the dosage of adsorbent, the pH of the solution, and the content of fluoride in the water.The Freundlich and Langmuir isotherm models were employed for the determination of the fluoride adsorption capacity, with the Langmuir model exhibiting the most favorable agreement.The temperature dependence of adsorption necessitates the assessment of thermodynamic parameters to ascertain the viability and spontaneity of the adsorption process inside the system.The study presents a thorough thermodynamic analysis, as well as information on how it performs in real water and its reusability as an adsorbent.The use of positively charged PCt for negatively charged fluoride ions removal is the first ever reported.This recent study has concentrated on producing value-added materials from waste toner without polluting the environment.Scheme 1 shows the overall story of this article.

Materials
The powder material was extracted from used exhausted printer cartridges (12 A type).All of the chemical reagents used in this work, such as sodium hydroxide (NaOH), sodium fluoride (NaF), and sulfuric acid (H 2 SO 4 ), were bought from Sigma Aldrich and used without additional purification.All experiments were carried out in deionized water.

Materials characterization
The substance's precise composition was determined by conducting X-ray fluorescence spectroscopy (XRF) using a Bruker India Scientific XRF Model S8 Tiger and powder samples placed on a 40 mm diameter pallet.Diverse sophisticated analytical techniques were employed to ascertain the morphological and physicochemical characteristics of the materials.The investigation of the materials' crystalline phase was conducted utilizing a Bruker AXS with D8 advance and a Cu Kα radiation source (λ = 1.5406).The Bruker Alpha II spectrometer was used to record Fourier transform infrared (FTIR) spectra.The Raman spectrum was obtained with Technos equipment, namely the IndiRam CTR-300 model, equipped with a 20X lens and a 100-mW laser.The NOVA touch LX4 instrument was employed to determine the surface area and pore volume of the material by the N 2 adsorption-desorption isotherm technique.The morphology of the materials is examined using field emission scanning electron microscopy (FESEM).The X-ray photoelectron spectroscopy (XPS) examination was conducted utilizing a Thermo Fisher Scientific-Nexsa surface analysis spectrometer that has a micro-focused, monochromatic Al K (1486.7 eV) X-ray source.

Batch studies
The fluoride removal process was investigated using the batch adsorption approach.The fluoride solution was made by combining sodium fluoride (NaF) with deionized (DI) water.The experiment was to mix 100 mg/L of fluoride solution with 0.2 g of PCt powder in a 250 ml conical flask.The experiment was conducted under neutral circumstances with a pH of 7. Subsequently, the flask was positioned in a rotary shaker and subjected to a speed of 150 revolutions per minute for a duration of 60 minutes.Subsequently, the material and solution were segregated, followed by the utilization of a spectrophotometer to ascertain the ultimate fluoride concentration.The equation provided was utilized to ascertain the fluoride adsorption capability of the material: .
Where q e stands for a material's adsorption capacity, C 0 for the material's initial fluoride concentration (mg/L), C e stands for the material's final fluoride concentration (mg/L), V for the solution's volume (liter), and m for the material's mass (gram).
The fluoride concentration was double-checked by Indian Standard guideline IS: 3025-60 (2008).The fluoride removal percentage was calculated using the following equation: .
The adsorption isotherm was used to better understand the adsorption process.The Langmuir adsorption isotherm is a popular model for explaining adsorbate-adsorbent equilibria.

Column studies
The fixed bed column study was carried out for the investigation of the material's industrial-level performance.For this experiment, a borosilicate glass column with a 25 mm diameter and 350 mm length was filled with sand, cotton, and PCt material.In this experiment, a 3-ppm fluoride concentration was taken.Because this will provide more practicality being applicable at the industrial level.In this experiment, the flow of water was kept downwards from the top of the column, and treated water was collected from the bottom of the column.Thereafter samples were collected at certain intervals to analyze the fluoride concentration.This experiment was done till its concentration reached 99% of its initial concentration.

Elemental composition
The XRF analysis of PCt materials before fluoride uptake revealed that the sample's main elemental components are Fe 3 O 4 , and SiO 2 , with traces of other elements, like magnesium, aluminum, iron, and zinc.According to pre-reported literature printer cartridge black toner powder has 35-40% carbon in it.The obtained waste toner powder results show a high iron oxide (magnetite) content (Table 1).Because XRF can't measure the amount of carbon, here in the table only metal oxide amount in % is mentioned (Fernández et al., 2022).

Structural characterization
To better understands materials chemical and physical properties, its further characteristic study was done and mentioned below.The X-ray diffractogram in Fig. 1(a) clearly indicates magnetic Fe 3 O 4 to be the dominant crystalline phase in the toner powder.The diffraction peaks at 2θ = 30.1º,35.4º, 43.0º, 53.4º, 56.9º, and 62.5º were indexed to the cubic structure of Fe 3 O 4 as confirmed by JCPDS 19-0629 (Zhu et al., 2018).It has a cubic inverse spinel group structure with space group of Fd3m (227).Since it has a cubic structure its cell parameters are a=b=c=8.39and its angles are α=β=γ=90º.It has the structure [Fe 3+ ] A [Fe 2+ Fe 3+ ] B O 4 , where A is the tetrahedral and B is octahedral, where this unit cell contains 32 Oxygen in a close-packed FCC (face-centered cubic) structure.Other components like SiO 2 (JCPDS No. 29-0085) and carbon black (JCPDS 41-1487) are found at planes ( 220) and (311) (Lou et al., 2013;Peng et al., 2013).As XRF results confirmed PCt has Fe 3 O 4 , SiO 2 , and carbon black.The broad peak at plane (002) represents the amorphous organic residue of the toner powder (i.e., the polymeric resins) with an amount of very low SiO 2 (Dong et al., 2017).
Since the PCt powder has organic contents in it confirming the type of unequivocal identification of the polymeric composition is difficult.Therefore, by FTIR analysis it is possible to identify various functional groups present in the PCt powder.In this view, the FTIR spectroscopy was recorded in the range of 400-4000 cm − 1 .Fig. 1(b) shows the presence of band at 3024 cm − 1 appears due to CH 2 asymmetrical stretching vibrations (Yordanova et al., 2014).The band at 2841 cm − 1 and 2910 cm − 1 corresponds to symmetrical stretching of CH 3 and CH 2 (Kumar et al., 2018).Another band at 1722 cm -1 and at 1597 cm − 1 corresponds to the C --O, stretching vibrations of the carboxyl group (Pirela et al., 2015).The bands at 1447 cm − 1 , and 1373 cm − 1 appear due to the absorption vibration of C-H stretching and deformation vibrations of the C-H group respectively.The band at 1147 cm − 1 is a characteristic of C-O-C stretching vibration in the repeated -OCH 2 CH 2 unit.The band at 1067 cm − 1 corresponds to the Si-O bonding of SiO 2 .The C-O stretching is linked to the intensity of the peaks at 1024 cm − 1 , and the existence of carbon and oxide may cause the C-O-C symmetric stretch, which is responsible for the band at 903 cm − 1 (Itoua et al., 2023).The band at 747 cm − 1 is caused by OH vibrations, whereas the band at 562 cm − 1 is caused by the Fe-O bond of Fe 3 O 4 .The presence of organic compound in the material is confined as polypropylene ((C 3 H 6 ) n ), its mixed in toner powder as binding material (Hammani et al., 2019).Other notable thing is C-C bonds in spectra, they correspond to the presence of carbon black in toner powder.
Fig. 1(c) shows the Raman spectra of PCt powder, this spectrum initially has three bands at 190 cm − 1 , 338 cm − 1 , and 479 cm − 1 , which corresponded to the vibration modes of Fe-O bonds of Fe 3 O 4 (Hai et al., 2008).The spectra showed a peak at 1278 cm − 1 that indicated the D-band of Fe 3 O 4 (Hai et al., 2008).The band at 883 cm − 1 and 995 cm − 1 corresponds to polypropylene. Figure S1 shows band at 1354 cm − 1 and 1582 cm − 1 corresponds to carbon black (Koochakzaei et al., 2022).All the data of Raman spectra confirms the presence of carbon and magnetic iron composite in it, which has good agreement with other characteristic data.
Fig. 1(d) displays the nitrogen adsorption-desorption isotherm of PCt, as indicated by BET data.The adsorption capacity of PCt has shown a gradual rise throughout the P/P0 range of 0.2-0.8,indicating the presence of mesopores.The PCt material exhibits a pore diameter distribution centered at 2.7 nm, as indicated by the pore size distribution obtained using the Barrett-Joyner-Halenda (BJH) model (Fig. 1(d) inset).This finding closely aligns with the literature findings.PCt has a pore volume of 0.004 cc/g and an estimated total specific surface area of 20 m 2 /g.It is important to note that PCt's surface area and mesoporous properties enhance the adsorption process.Further, the physical characteristics of PCt are summarized in Table 2.
The morphology of the adsorbent was studied using the FESEM (Fig. 2(a, b)).The adsorbent's surface morphology shows a cottonlike structure with many spheres and a rough surface with numerous bumps on the surface (Kaipannan et al., 2019).Additionally, the elemental mapping studies and EDAX spectra of the PCt material confirmed the existence of C, Fe, O, and Si (Au because of coting on

Adsorption results
The fluoride adsorption capacity of PCt was assessed at an initial fluoride concentration of 100 mg/L, and experimental results show that the capacity is 60 mg/g towards fluoride.ICP-MS testing was done on this solution to ensure that there was no elemental leaching present (Table S1).To investigate its overall potential towards fluoride ions, additional experiments such as isotherm, kinetic, and thermodynamic studies were carried out.

Variations in the fluoride concentration and adsorption isotherm
To examine material performance in different fluoride concentrations, this study was done and with these results, its isotherm's parameters were also calculated to further know about material's maximum adsorption capacity.To perform these experiments, we took fluoride solution in the range from 10 to 100 mg/L, with 0.02 g/L mass, and shaken for 60 minutes at neutral pH.The percentage of adsorption considerably decreased as the initial fluoride concentration rose (Fig. 3).
Fig. 3(a, b) reveals that when the fluoride concentration was increases, the removal efficacy of the adsorbent material decreases.The current behavior is caused by increased fluoride concentration, which fills all active sites of adsorbent material.Between the surface of the adsorbent and the adsorbate, the adsorption process establishes a dynamic equilibrium (Raghav and Kumar, 2018).Adsorption isotherms like Langmuir and Freundlich isotherms are the widely used adsorption models for studying the mechanism of the adsorption process.The maximum adsorption capacity (q m ) obtained by fitting experimental data to Langmuir equations was found to be 65 mg/g (Table 3).The correlation coefficient (R 2 ) is 0.99, and the value of R L in this study is 0.9, indicating that fluoride adsorption on PCt particles is favorable in nature.The Langmuir equation can adequately explain this adsorption process, as shown by the fact that the  correlation coefficient (R 2 ) obtained from the Langmuir model is greater than that obtained from the Freundlich model.The adsorption capacity K f for the Freundlich isotherm is not very near to the experimental value because it depends on the adsorbate and adsorbent's properties.The nonlinear parameter n represents the heterogeneous energy of the adsorption surface.In our case, the Freundlich equation was satisfied if 1/n < 1, which indicates that the adsorption is likely to be successful.

Effect of adsorbent dose
PCt powder doses of 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g/L were used to study about how much amount of adsorbent should be suitable for specific concentration of fluoride.To perform this experiment 100 mg/L of fluoride with 100 ml volume was shaken at 150 rpm in neutral condition, with a contact time of 60 minutes.The effects of PCt adsorbent dosages on fluoride uptake are shown in Figure S2.
The graphs demonstrate that as the adsorbent dosage was increased from 0.2 to 1.2 g/L, the removal efficiency of fluoride rose quickly.Further increases in material dose did not significantly boost the elimination of fluoride.The enhanced fluoride removal capacity could have been caused by the increased material dose, which may have resulted in more sorbent surfaces or active sites for fluoride uptake (Rathore and Mondal, 2017).But beyond a certain point, it stops showing an upward trend since the amount of fluoride ions present is the same but the number of active sides is greater; as a result, after 0.08 g of adsorbent, a straight line is visible.

Impact of contact time and adsorption kinetics
The performance of the material's adsorption is also influenced by the contact time.To study more about, how much time is sufficient for the better fluoride adsorption into PCt, experiments were done in the time range of 20-120 minutes.The tests were done with 100 mg/L of fluoride, a pH of 8, and a mass of 0.02 g/L (Fig. 4(a)).For porous solid adsorbents, the total adsorption rate is determined by how well the adsorbate molecules can get inside the particle.The process of adsorption was studied to find out how fast and where the adsorption happened up to equilibrium.The pseudo-first-order model, the pseudo-second-order model, and the intraparticle diffusion model were all used to show how the adsorption happened (Fig. 4).
Fig. 4(a) demonstrates that fluoride adsorption onto PCt occurs quite quickly and reaches equilibrium in less than 120 minutes, suggesting that the mesoporous structure and positively charged surface would speed up this process due to electrostatic attraction.The experimental results and calculated data in Fig. 4(b) and (c) demonstrate that the pseudo-second order model best fitted for fluoride removal.The theoretically calculated adsorption capacity q e = 67 mg/g, was very close to the experimental value q e = 60 mg/ g.Another important factor is the linear fitting coefficient (R 2 = 0.99) of this graph shows high value when comparing to pseudo first order model (Table 4).
To learn more about the adsorption rate, the kinetic data was fitted to the intra-particle diffusion model (Fig. 4(d)).The results show that fluoride adsorption on PCt occurs in two steps, the first slope is sharp compare to second, indicating that initial slop is more responsible for fast adsorption process and remaining process occurs in second step.Based on published data, it has been shown that the initial linear slope corresponds to a rapid diffusion process occurring due to boundary effects, while the subsequent linear slope represents a slower attainment of equilibrium resulting from intra-particle diffusion processes.The high fitted correlation coefficients of R 2 (0.99) indicate a strong relationship between fluoride adsorption and the intra-particle diffusion model.This suggests that the adsorption process of fluoride onto PCt can be better explained by the intra-particle diffusion model, as shown in Table 5.
Fluoride's higher K d value (1 mg/g/min 1/2 ) indicated a more rapid diffusion into PCt's mesoporous structure.It showed a stronger boundary layer effect at first slope (i.e., molecular diffusion in solution) during the adsorption of fluoride by PCt.

Impact of the solution's pH
Another key aspect is pH of the solution which directly affects the absorption process of adsorbent materials.In order to better understand the adsorption mechanisms in both acidic and basic environments, further experiments from pH range 4-9 were performed with 0.2 g of adsorbent dose and initial fluoride concentration of 100 mg/l, and contact period of 60 minutes at room temperature.The adsorbent's surface charge and the pollutant's solution phase chemistry both play essential roles in the adsorption process.Since groundwater normally has a pH between 6 and 8, the aforementioned pH range was taken into consideration (Fig. 5(a)).
The point of zero charge value (pH PZC ) of adsorbent material is essential to the adsorption process.The pH PZC value of adsorbent indicates that the adsorbent surface's net charge is zero.When the pH of the solution falls below pH PZC , the adsorbent's net surface charge on the solid surface becomes positive due to excess of H + ions, which is advantageous for fluoride adsorption because fluoride is a negatively charged ion that is deemed to show coulombic attraction.In this case, the PCt has neutral response around pH 7.5, suggesting that the surface of the material is positively charged below this, which facilitates fluoride ion adsorption at lower pH levels (in the presence of H + ions).Fig. 5(b) demonstrates how the percentage of fluoride adsorption reduces as the pH of the solution rises because the fluoride is repelled by the adsorbent's surface negative charge (rising OH -ions).Similar findings have been reported in the

Table 3
Isotherm parameters for fluoride adsorption on PCt.

Langmuir model
Freundlich model

Table 4
Parameters of pseudo-first-order and pseudo-second-order kinetic models.

Pseudo-first-order model
Pseudo-second-order model K f q e R 2 K s q e R 2 PCt 0.08 49 mg/g 0.93 0.0057 67 mg/g 0.99

Table 5
Parameters for intraparticle diffusion of PCt.literature, where the increased fluoride absorption at lower pH was attributed to the electrostatic attraction process.Although water's pH typically has a neutral value, we conducted all of our experiments at pH 7.

Adsorption thermodynamics
A thermodynamic study was done to determine the adsorption capacity at various temperatures, despite the fact that temperature affects the adsorption process.On the other hand, room temperature reactions are more useful in investigations of contaminant removal.The purpose of this study was to learn more about the effects of temperature throughout the year.For the adsorption experiments, three different temperatures (303, 313, and 323) were used together with a starting fluoride concentration of 100 mg/L (at neutral pH with 0.02 g adsorbent dose and 60 min.contact period).Indicating that PCt interacts with fluoride at a little lower temperature, it appears that the adsorption capability steadily reduced as the temperature increased.Table 6 The study's results suggest that doing the adsorption process at lower temperatures, particularly at room temperature, offers more advantages.This finding is substantiated by the ΔG values acquired under three distinct temperature settings.The confirmation of the reaction's feasibility and the spontaneous nature of adsorption was achieved by the detection of a negative change in Gibbs free energy (ΔG) at a given temperature (Raghav et al., 2018).The correlation between temperature and the change in Gibbs free energy (ΔG), as explained in the section on activation energy, illustrates a decreasing trend in the viability of fluoride adsorption.The reported ΔH value of -14.7 kJ/mol supports the practicality and appropriateness of the adsorption approach, indicating that the interaction between fluoride and PCt may occur at a much lower temperature.The symbol ΔS represents the degree of affinity and unpredictability found at the boundary between a solid and a liquid, which aids in the adsorption process.This discovery is consistent with the previously published statistics.Physical adsorption is a phenomenon that causes a decrease in both the free energy and entropy of the adsorption system, resulting in an exothermic reaction.

Activation energy
An Arrhenius plot was created after the kinetic data had been collected and the rate constant for the reaction at the three distinct temperatures was identified.To determine the process's activation energy The log k 2 (adsorption capacity) vs. 1/T (temperature) plot is produced.
The values of Ea (activation energy) were determined from the slope of the plot presented in Figure S3.The activation energy value helps to explain the mechanism whether the binding in adsorption occurs through physisorption or chemisorption.The activation energies for physical attraction are typically lower.While chemisorption has higher activation energies and requires more energy (Raghav et al., 2018).According to data from activation energy studies, fluoride ions onto the PCt binds through physisorption since its ions have activation energies under very low (less than 40 kj/mol) at room temperature.The high fluoride affinity of PCt and the viability of the reaction are supported by the much lower value of the activation energy Ea (19.2 kJ/mol) mentioned in Table 7.

Fixed-bed column adsorption study
The effect of fluoride concentration on material was studied with this method.For this experiment, PCt material was put into a column with sand and cotton on top and bottom to ensure ideal packing.This column has a bed length of 23 cm, 25 g of PCt material, and 250 µm homogeneous sand particles.The obtained results are mentioned in Table 8, 3 ppm fluoride results show that it can treat large volume of fluoridate water.these results indicate that lower fluoride concentration gives higher breakthrough time and larger Veff value (Talat et al., 2018).
Because flow rate is a little higher it shows saturation on the material in the early stage, more contact time provides more reaction time.Although, 3 ppm concentration reaches its permissible limit after treating 9 liters of fluoridate water.Figure S6 shows the C t /C o versus time (t) graph of fixed bed column studies at different fluoride concentrations.As can be seen from Figure S6, column study data compliments the batch adsorption data in this study.In summary, we can say this could be used on a large scale to treat various concentrations, without elemental leaching.

Real sample analysis
A real groundwater sample collected from Himmatgarh Village, Dhar District, Madhya Pradesh, India, is utilized to evaluate the efficacy of the adsorbent in eliminating fluoride.The concentration of fluoride in the hand pump samples was measured to be 6.8 mg/ L, exceeding the allowable limit by approximately sixfold.For the treatment of 100 ml of water, a quantity of 2 g of PCt material was utilized.The adsorbent effectively reduces the fluoride concentrations in drinking water to levels below the authorized limit set by the World Health Organization (1.5 mg/L).Table 9 displays the properties of the water samples before and after treatment, demonstrating strong evidence that the fluoride concentration reduced to levels that are considered acceptable after the treatment material was applied.The fluoride concentration in the groundwater sample is 1.2 mg/L.Another significant finding is that the TDS and conductivity of the treated water remain constant, suggesting that no ions from the adsorbent have transferred into the water.This is a vital factor for the adsorbent, particularly in its implementation for the purification of potable water.

Regeneration and desorption studies
The spent PCt powder was regenerated with chemical treatments such as dilute H 2 SO 4 (1 M) and dilute NaOH (1 M) (as desorbent agents) followed by reported article of Choudhary and team (Choudhary et al., 2023).The regeneration studies were done at room temperature.The spent PCt powder was first dipped into the basic solution in 250 ml conical flask.The flask was then shaken for 60 minutes at room temperature with a thermomechanical shaker.After multiple washing, it was dipped into the acidic solution then the this PCt powder was washed several times with deionized water, and the resultant powder was heated in a hot air oven at 80 ºC for 2 hours.After regeneration it was again used to treat fluoride from water, after every recharge it loses its original potential may because some pores of adsorbent get saturated and does not regenerate again.The desorption efficiency was calculated as the ratio of fluoride desorbed to adsorbed.Fluoride desorption levels ranged between 87% and 88% after its first regeneration.After three cycles, desorption efficiency decreased by 25-27%.The desorption mechanism, we believe relies on the principal of the ion exchange method.On the adsorbent surface, when the fluoride-saturated media is treated with 1 M NaOH, OH⁻ ions of the NaOH replaces the adsorbed fluoride ions, once the PCt surface gets OH⁻ saturate than the treatment with acid is done to make surface neutral so that adsorbent gets ready to remove more fluoride ions present in the walls.Figure S5 depicts the desorption and reuse of PCt at fluoride concentration at 10 mg/L throughout three regeneration cycles.its adsorption capacity decreasing from 65% to 40%, during the three-regeneration process.Based on the regeneration results, PCt would be an efficient and cost-effective adsorbent material with good regeneration ability.As a result, the PCt will be reused for up to three regeneration cycles.PCt was applied after regeneration and demonstrated no leaching in water, demonstrating its stability and reproducibility (Table S2).

Characteristic analysis after adsorption
After fluoride adsorption materials characteristic analysis is very important aspect of the study, following that spent PCt powders morphology studies were carried out, in Fig. 6.
As discussed, earlier PCt powder has cotton like structure, and sphere of Fe 3 O 4 .After fluoride adsorption it's does not shows any drastic morphological differences between bare and spent PCt powder.Its EDX results clearly shows that it has fluoride (marked in circle) on its surface, which supports successful adsorption of fluoride on PCt (Figure (c)).Its elemental mapping on spent PCt shows it has same chemical composition as bare PCt (Figure (d-i)), indication that, during adsorption process PCt powder has not loses its originality.After studying spent PCt morphology, its characteristic properties before and after adsorption was also studied which mentioned in Fig. 7 to better understand the materials importance as adsorbent.
As can be seen from Fig. 7(a), XRD spectra of material before fluoride adsorption and after adsorption, which indicates that there is  no major difference presented in it.This XRD spectrum is matched well with the reported literature.Furthermore, after fluoride adsorption, PCt XRD revealed that the broad peaks (plane 002) of organic residue have completely disappeared may due to the decomposition of polymeric resin in it, or can its reflections are missing after annealing because the grains are not oriented correctly to allow diffraction from these planes.To confirm polymeric resin involvement, FTIR data (Fig. 7(b)) of material before and after adsorption was compared.However, this data does not show any major difference in it, which is directly attributed to that its polymeric resin does not interrupt the adsorption process.Although its carbon silica and iron content remain as it is, which strongly supports that it did not leach anything during adsorption.In Fig. 7(c), Raman spectra of material before and after adsorption show that it's surface after adsorption changes a little due to the presence of fluoride ions on its surface, but does not support any chemical change.To confirm this X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the chemical compositions and electronic structure of PCt, and a typical full XPS spectrum is shown in Fig. 7(d).Fig. 7(d) shows the XPS survey spectra of bare or spent material.As shown in the wide scan XPS spectra, the characteristic peaks of the main elements (C, Fe, Si, O) of PCt were detected.Following fluoride adsorption, a new peak at 685 eV grew, which was attributable to the F 1 s photoelectron.The appearance of a F 1 s signal in spent PCt proved that the surface interaction between fluoride and PCt material was taking place.This strong F1s peak shows PCt has a huge amount of fluoride ions on its surface after adsorption.Figure S6 (a) displays the high-resolution Fe 2p spectrum.the Fe 2p 3/2 and Fe 2p 1/2 spin-orbit peaks, which are located at binding energies of 710.8 eV and 724.6 eV, respectively, indicate the formation of Fe 3 O 4 (Bhaumik et al., 2011).The C 1 s high resolution spectra (Figure S6 (Saini et al., 2019).After adsorption PCt shows shifting in its characteristic peaks.This shift in XPS spectra occurs because increasing the electron density of an atom (such as fluoride ion compared with Oxygen) decreases the binding energy.To confirm this, Figure S7 compares the normalized O 1 s spectra of the composite before and after fluoride adsorption.It is evident that there is a notable decrease in the intensity of O 1 s peaks, indicating that fluoride in the water is exchanged with hydroxyl ions on the adsorbed surface, in accordance with Dhillon et al. (2016).
Table 10 shows the different adsorbents and their fluoride adsorption capacity (mg/g), which includes various metal oxides and carbon-based materials developed till date.This clearly indicates that our material shows excellent results without any synthesis efforts and costs.
The possible mechanisms of fluoride adsorption on PCt powder are depicted in Fig. 8.For the remediation of fluoride-containing water, metal oxides and hydroxides as well as their composites have received quite a bit of attention because they have positively charged surfaces that make it easier for negatively charged fluoride ions to be attracted to them via electrostatic attraction.It is worth noting that the majority of the literature on the adsorptive mechanism of fluoride focuses on the adsorbent surface.According to the reported hypothesis, Carbon black is insoluble in water, when mixed with metal oxides its water dispersibility increases, which gives more active site to fluoride for adsorption.Fluoride adsorption mechanism onto PCt is presented in Fig. 8.As can be seen from Fig. 8, it can be understood that adsorption of fluoride on materials surface is majorly physical adsorption.Because this material has mesoporous properties it generates more active sites and provides more pore diffusion on it.At certain extent this study found that the PCt adsorbs fluoride through ion exchange, and surface complexation, by hydrogen bonding processes.Research suggests that the chemisorption of a single layer of molecules on an adsorbent surface is typically followed by further layers of physically adsorbed molecules from the adsorbate.The physisorption process is dominating in the adsorption process because the presence of carbon provides an abundance of surface complexation and electrostatic attraction to metal ions.Hence, the improved adsorption behavior can be explained by the physio-chemical interactions of fluoride species with PCt surface.

Table 10
The adsorption capability of various metal oxide, carbon black/carbon-based materials, and metal-carbon composite material adsorbents used for fluoride removal is presented.

Conclusion
The performance of the adsorption onto PCt powder was studied in the current study.At a pH of 7.0, with a contact period of 60 minutes and an adsorbent dose of 0.2 g, the adsorbent exhibited a percentage adsorption of 62% and a maximum adsorption capacity of 65 mg/g.The present study examined the equilibrium, kinetics, and thermodynamics of fluoride adsorption.The equilibrium data was analyzed using the Freundlich and Langmuir isotherm models, revealing a maximum adsorption capacity of 65 mg/g.The PCt powder has a morphology like cotton, accompanied by the presence of Fe 3 O 4 spheres dispersed throughout.The rough and uneven surface of the material offers an increased number of active sites for the adsorption of fluoride.The adsorption process was found to conform to the pseudo-second-order model, as determined by kinetic investigations with PCt.This conclusion was supported by the close agreement between the theoretical and experimental sorption capabilities, as evidenced by a high coefficient of determination (R 2 = 0.99).After fluoride adsorption its characteristic data shows that it has no change it its structure and chemical composition, also it does not cause any elemental leaching in water.It's can recharge and reuse 3 times after fluoride adsorption.This study shows that how waste can be used to treat pollutant and can be reused multiple times.Fluoride treatment with PCt does not charges any expense and gives really great result for water treatment.Exhausted printer cartridges toner powder is first ever reported for fluoride treatment, is best example of waste to wealth research.

D
.Choudhary et al.
material) elements on the material surface (Fig.2(c)).To learn more about the distribution of C, Fe, O, and Si in the matrix as seen in Fig.2(d) and (e), respectively, X-ray elemental mapping of the PCt material (Fig.2(d-i)) was carried out.The presence of carbon ferric composite is confirmed by the intermixed distribution of the elements carbon, iron, silica, and oxygen.

Fig. 3 .
Fig. 3. shows the equilibrium concentration and adsorption capacity in (b) and the initial concentration of fluoride vs removal % in (a).Adsorption curves for the Freundlich and Langmuir isotherms (c, d) of the PCt, respectively.

Fig. 5 .
Fig. 5. ΔpH curve of PCt (a) and varying pH effect on the removal of fluoride (b).
(b)) show peak at 284.6 eV corresponds to C-C bond of carbon black.The O 1 s spectrum (Figure S6 (c)) shows binding energy at 533.1 eV corresponds to oxygen iron bond.After fluoride adsorption the binding energy of O 1 s shifted a little may because presence of fluoride on PCt surface affect its position.In high-resolution XPS spectra (Figure S6 (d)), peak at 103 eV and 154 eV corresponded to Si 2p and Si 2 s respectively, confirm the presence of SiO 2 in PCt.Hence its confirmed that bare and spent PCt powder has major part of carbon/iron composite in it
D.Choudhary et al.

Table 2
Physical properties of PCt sample.
D.Choudhary et al.

Table 6
Thermodynamic studies of PCt for defluoridation.

Table 7
Activation energy parameters of PCt.

Table 8
Adsorption data for fixed-bed PCt column for fluoride adsorption at 3 ppm fluoride concentration.

Table 9
Raw water and PCt treated water parameter.